11. TRACE FOSSILS IN DEEP SEA DRILLING PROJECT LEG 58 CORES

A. A. Ekdale, Department of Geology and Geophysics, University of Utah, Salt Lake City

ABSTRACT

Bioturbation is extensive in all the facies cored during DSDP Leg58 in the Philippine Sea, and cores not disturbed by drilling containseveral distinctive trace-fossil types. Terrigenous turbidites show lit-tle, if any, bioturbation in their lower sub-units, although the upper,fine-grained sub-units commonly contain burrowed horizons. Pelag-ic biogenic oozes contain a typically deep-sea trace-fossil assemblagedominated by Planolites, Zoophycos, and Chondrites. Pelagicbrown clays contain an abundance of smeared and deformed bur-rows, suggesting deposition of low-strength sediment at or below thecarbonate-compensation depth.

INTRODUCTION

Trace fossils are biogenic sedimentary structures,such as animal tracks, trails, burrows, and borings.They are evidences of the presence and activities of an-cient organisms; thus, they often provide important in-formation about the paleoecology of benthic creaturesthat may or may not be preserved as body fossils. More-over, trace fossils are always in situ (it is rarely possibleto transport a sedimentary structure), and so they are ofconsiderable utility in interpreting sedimentary facies.Trace fossils have been employed in various deposi-tional settings as (1) paleobathymetric indicators (e.g.,Seilacher, 1963, 1967; Rodriguez and Gutschick, 1970;Chamberlain, 1971; Kern and Warme, 1974; Ekdale,1978b); (2) indicators of current direction (e.g., Sei-lacher, 1955; Birkenmajer and Bruton, 1971; Schàfer,1972); (3) keys to understanding erosion-redepositionevents (e.g., Seilacher, 1962; Goldring, 1964; Bromley,1967, 1968, 1975; Howard, 1978); (4) indicators offresh-water or marine depositional environments (e.g.,Howard et al., 1975; Mayou and Howard, 1975; Cham-berlain, 1975); (5) indicators of the oxygenation of bot-tom waters (Rhoads and Morse, 1970; Kauffman,1977); and (6) reflections of important sediment proper-ties, such as chemical composition (e.g., Kennedy, 1975;Harrington, 1978), grain size (e.g., Sanders, 1958; Pur-dy, 1964; Rhoads, 1967, 1970; Warme, 1967), and shearstrength (e.g., Kennedy and MacDougall, 1969; Bergerand Johnson, 1976; Ekdale and Berger, 1978).

The importance of trace fossils in deep-sea sedimentslies in the fact that body fossils of benthic macro-invertebrates inhabiting the abyssal realm are exceeding-ly rare. With few exceptions (e.g., Cheetham, 1975;Kauffman, 1976), trace fossils constitute almost the to-tal of our paleontological information about the bot-tom-dwelling megafauna in ancient deep-sea deposits(Ekdale, 1978a, 1978b). The kinds of tracks and trailsproduced on the deep-sea floor typically are very dis-tinctive and therefore are often cited as indicators of

abyssal depositional conditions (Bourne and Heezen,1965; Heezen and Hollister, 1971; Hollister et al., 1975;Ekdale and Berger, 1978; Kitchell et al., 1978). Thesesediment-surface trails and related structures have beenobserved as bedding-plane trace fossils in ancient deep-water deposits, particularly turbidites, exposed on land(Seilacher, 1962, 1967, 1974; Ksiazkiewicz, 1970; Kern,1978). However, continuous pelagic sedimentation, ac-companied by continuous biologic reworking of the sed-iment, generally precludes the formation of beddingplanes. In fact, surface tracks and trails, as well asshallow infaunal burrows (in the upper few centimetersof sediment), have little chance of preservation, becausethey are continually destroyed by the intense bioturba-tion of deep-burrowing infauna in the deep sea.

Berger and Heath (1968) developed a quantitativetheoretical model of reworking of sediment by benthicorganisms, in which the surface layer of sediment(called the "mixed layer") is a zone of instantaneous,homogeneous mixing of sedimentary particles. Usingabyssal box cores from the equatorial Pacific, Berger etal. (1979) demonstrate that a mixed layer does indeedexist. It is a layer some 5 to 8 cm thick in which intensebioturbation virtually homogenizes the sediment, andindividually distinct burrows and any stratificationfeatures are not visible. Below the mixed layer, todepths of 20 to 35 cm below the water-sediment inter-face, lies the "transition zone." In this zone active bur-rowing also occurs, but at a rate far lower than in themixed layer. Burrows produced in the transition zoneinclude those which eventually are preserved in the"historical layer," which is the horizon directly under-lying the transition zone. The upper boundary of thehistorical layer is the level below which no new burrowsare produced. In calcareous oozes from low latitudes intoday's oceans, that level is approximately 20 to 35 cmbelow the sediment surface.

Thus, the typical deep-sea ichnofacies that character-izes pelagic oozes (i.e., deposits far removed from theinfluence of continents) includes only infaunal traces

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which happen to be produced deep in the sediment be-low the mixed layer. These include the ichnogeneraChondrites, Planolites, Skolithos, Teichichnus, andZoophycos (Ekdale, 1978a, 1978b; Ekdale and Berger,1978). On the other hand, the ichnofacies that charac-terizes abyssal clastic material, such as that deposited atregular or sporadic intervals on a submarine slope, com-monly includes such surface and shallow infaunalichnogenera as Helminthoida, Nereites, Spiroraphe, andPaleodictyon (Seilacher, 1967; Kern, 1978). Both "deep-sea" ichnofacies may be formed at the same depths;their distinction is primarily in preservation.

TRACE-FOSSIL FACIES

Lithofacies cored in the Philippine Sea during DSDPLeg 58 include (1) hemipelagic clay and mudstone; (2)resedimented mudstone, sandstone, and conglomerate;(3) resedimented carbonate; (4) pelagic non-biogenicsediment; (5) pelagic biogenic calcareous and siliceoussediment; and (6) pyroclastic material (see lithologicsummaries in other chapters of this volume). Theselithofacies bear characteristic suites of trace fossils andbioturbation features (i.e., ichnofacies). Detailed de-scriptions of the common ichnogenera are given byChamberlain (1975), Hantzschel (1975), and Ekdale(1978a, 1978b).

Hemipelagic and resedimented terrigenous facies,such as those in the upper parts of Holes 442, 443, 444,and 446, include materials derived from shallower wateror land and deposited in pulses by turbidity currents orsome similar mechanism. Typically, these deposits ex-hibit normal "Bouma sequences" (Bouma, 1962) grad-ing from coarse, often conglomeratic sediment withNummulites at the base into fine-grained material at thetop of each turbidite unit. The lower, coarser sub-unitsof the cycle include graded bedding, convolutions,flame structures, etc., but no burrows. In these coarse-grained deposits one might expect to see occasional"escape structures" (Schüfer, 1972) produced by organ-isms digging out of a newly deposited turbidite. How-ever, such structures have been cited only rarely in theliterature (Hantzschel, 1975), and none were discoveredin Leg 58 cores.

The upper, fine-grained sub-units of the turbidites inLeg 58 cores typically are finely laminated and unbio-turbated through most of their thickness. However,some burrowing does occur in the uppermost sub-unitsof many sequences, which display no laminations, butcommon Chondrites. Middle-Eocene deposits of Hole445 typify this pattern (Plate 1, Figures 1 and 2). Thewell-defined burrowed horizons sometimes are only 1 or2 centimeters thick, although the entire turbidite may bemore than half a meter thick. The Chondrites in thesefine-grained sub-units is typically small, with individualtunnels about 1 mm in diameter. Branching is primarilyhorizontal or subhorizontal, suggesting that organicmaterial usable as food was concentrated along lamina-tion planes rather than distributed uniformly through-out the hemipelagic mud.

Pelagic biogenic facies, particularly the carbonates ofHole 445, contain the richest trace-fossil assemblages.This is no doubt due to the fact that calcareous and sili-ceous oozes generally are deposited beneath moderatelyproductive regions of the oceans, which provide a con-tinuous supply of organic matter to the bottom. Highbenthic biomass characterizes these regions, and in-faunal members of the benthic communities are com-mon and fairly diverse. Burrowers that homogenize thesediment live in the upper few centimeters (the mixedlayer); record-making burrowers live somewhat deeperin the sediment (Berger et al., 1979).

The trace fossils characterizing the biogenic-oozefacies in Leg 58 cores include Chondrites, Planolites,Skolithos, and Zoophycos. This association is particu-larly well preserved in upper-Miocene deposits of Hole445 (Plate 1, Figures 4, 5, and 6). These ichnogenera aresuperimposed on wholly bioturbated sediment, so theyare thought to have been introduced below the mixedlayer, in the transition zone. Chondrites typically occursin small patches and commonly cuts across all otherburrows present. Unlike the Chondrites in hemipelagicterrigenous deposits, those in pelagic ooze commonlybranch at an oblique angle to the horizontal and varyfrom 1 to 2 mm in the diameter of the individual tun-nels. Planolites includes the relatively large (0.5 to 2.0cm in diameter), unbranched, horizontal or sub-hori-zontal burrows in the cores. These are very common inall burrowed facies, but are especially abundant in thebiogenic ooze. Skolithos resembles Planolites in virtual-ly all respects except orientation; it is vertical or sub-vertical, and its chances of being recovered in a DSDPcore are thus small. Skolithos occurs in very few Leg 58core sections. Zoophycos is the most complex and dis-tinctive burrow in the cores, and it is very common inthe biogenic facies. Typically it is pelleted, suggestingthat the burrow system was stuffed with fecal pellets(see also Ekdale, 1978a). Although the possibility existsthat at least some Chondrites, Planolites, and Skolithoswere maintained as open dwelling structures during thelives of their creators, there is little doubt that Zoophy-cos was a complex deposit-feeder burrow system thatwas continually filled in as the burrowing organismmoved through the sediment.

The facies of pelagic brown clay, exemplified in thelower part of Hole 442B, include sediments deposited ator below the carbonate-compensation depth (CCD)beneath low-productivity regions in the oceans. Onemight suspect that the extremely low sedimentationrates (roughly 1 mm/1000 yr) and meager supply oforganic matter to the bottom would severely limit theoccurrence and abundance of benthic life in abyssalclays. Benthic biomass is indeed low — generally one ortwo orders of magnitude lower than that in biogenic-ooze regions (Zenkevitch, 1970) — but bottom-dwellingepifauna and burrowing infauna do occur in abyssal-clay regions. Distinctive burrows preserved in pelagicclays in DSDP cores are less diverse than those in otherfacies, although the intensity of the bioturbation is com-

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monly just as high. The trace fossils characterizingbrown abyssal clays in Leg 58 cores are dominated byPlanolites. The low carbonate content of pelagic claytypically leaves it with little competence. Shear strengthis much lower than in calcareous deposits, and burrowsare much more easily deformed and (or) destroyed bysediment flow. Thus, Planolites and other pelagic-clayburrows are commonly smeared; the edges of the bur-rows are pinched out and their margins are "frazzled"(Plate 1, Figures 3, 7, and 8). This aspect of burrowpreservation has been noted in box cores of moderndeep-sea sediment (Berger and Johnson, 1976; Bryant,1977; Ekdale and Berger, 1978; Berger et al., 1979) andis evident in Leg 58 cores, especially in the Miocene clayof Hole 442B.

DISCUSSIONThe "deep sea," which encompasses the abyssal and

hadal realms of the oceans, at depths greater than2000m (Bruun, 1957), imposes peculiar ecologic (in-cluding sedimentologic) conditions on its inhabitants.The lack of direct solar energy input precludes primaryproduction, so benthic communities typically are dom-inated by detritus feeders — some of which graze on thesea bottom, some of which feed in the sediment, and allof which are potential trace-fossil makers. These crea-tures are rarely very selective in what they eat, and manyare such trophic generalists that it is often difficult todistinguish between predator and scavenger (Daytonand Hessler, 1971). The absence of sunlight, seasonal-ity, and significant bottom currents in the deep seacreates a monotonous environment in which to live. Thebehavioral responses of the bottom dwellers to theseuniform conditions, as evidenced in their trace fossils,likewise are rather monotonous. Apparently, the behav-ior patterns of deep-sea burrowers have not changedsignificantly since the Cretaceous (Seilacher, 1974; Ek-dale, 1978a). Consequently, trace-fossil assemblages indeep-sea sediments are distinctive.

Paleobathymetry of DSDP core samples can be in-ferred from benthic fossils and (or) sedimentary struc-tures; trace fossils are both. The trace fossils preservedin Leg 58 cores aid our interpretation of the originalwater depth during deposition of the various lithofaciesin the Philippine Sea. The three major lithofaciesgroups present in Leg 58 cores (terrigenous turbidites,biogenic ooze, and pelagic clay) are certainly of deep-water origin, and each contains a diagnostic suite ofbioturbation features. Thus, there appear to be at leastthree different "deep-sea" ichnofacies.

In Seilacher's (1967) bathymetric zonation of ichno-facies, abyssal conditions are indicated by a diverse suiteof highly patterned and dominantly horizontal deposit-feeder trails and burrows. These compose his "NereitesIchnofacies" (after a common ichnogenus), which iscommonly associated with well-bedded strata, such asdistal turbidites. Although such clastic deposits arecommon in the deep sea, the characteristic trace fossilsare predominantly horizontal structures and thus arerarely recognized in small-diameter DSDP cores. Forthis reason no definite ichnogenera have been identified

in the turbidites recovered on Leg 58, with the exceptionof Chondrites, an infaunal burrow system not confinedto horizontal stratification planes. The bioturbated,hemipelagic, fine-grained sub-units of Leg 58 turbiditesare thought to have been deposited on (and probablynear the base of) a submarine slope. The bottom cer-tainly lay below normal shelf depths (i.e., several hun-dreds of meters), but whether or not the sediments weredeposited at truly abyssal depths is a question that can-not be answered by trace fossils alone.

The facies of pelagic ooze and clay probably weredeposited at abyssal depths, because their sites of depo-sition had to be far enough removed from the base ofthe continental slope to avoid any terrigenous influx.The trace fossils in the calcareous biogenic ooze arediverse, exhibit good color contrast, have sharp, well-defined margins, and commonly exhibit a vertical (aswell as horizontal) component of burrowing. The highcarbonate content of the calcareous ooze indicates thatit was deposited above the CCD. The trace fossils in thelow-strength brown clay are not diverse, exhibit poorcolor contrast, have fuzzy margins, and commonly aresmeared and stretched out; vertical burrows are ex-tremely rare. The very low carbonate content of the clayindicates that it was deposited at or below the CCD. Anapproximate bathymetric zonation of the three deep-seaichnofacies comprises, in order of increasing waterdepth, (1) the sharply defined bioturbated horizons ofhemipelagic mud containing Chondrites, (2) the Chon-drites-Planolites-Zoophycos association of biogenic ooze,and (3) the smeared Planolites of pelagic brown clay.

ACKNOWLEDGMENTS

This research was supported by National Science Founda-tion Grant No. OCE 75-21601. Peter H. Roth read the originalmanuscript and offered suggestions for its improvement.

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PLATE 1

Typical Trace Fossils in DSDP Leg 58 Cores(bar equals 1 cm)

Figure 1 Bioturbated horizons interbedded with laminated horizons in middle-Eocenehemipelagic mud. 445-74-2, 120-131 cm.

Figure 2 Bioturbated horizons interbedded with laminated horizons in the middle-Eo-cene hemipelagic mud. 445-74-2, 82-93 cm.

Figure 3 Deformed burrows with fuzzy margins in Miocene pelagic clay. 442B-2-5, 17-24 cm.

Figure 4 Chondrites and Planolites in upper-Miocene calcareous ooze. 445-28-1, 34-33cm.

Figure 5 Chondrites, Planolites, and Zoophycos in upper-Miocene calcareous ooze.445-25-6, 1-10 cm.

Figure 6 Chondrites, Planolites, and Zoophycos in upper-Miocene calcareous ooze.

445-23-1,4-13 cm.

Figure 7 Deformed, smeared Planolites in Miocene pelagic clay. 442B-2-5, 81-88 cm.

Figure 8 Deformed, smeared Planolites in Miocene pelagic clay. 442B-2-5, 60-67 cm.605